"....He earned most of his fortune in the steel industry. In the 1870s, he founded the Carnegie Steel Company, a step which cemented his name as one of the "Captains of Industry". By the 1890s, the company was the largest and most profitable industrial enterprise in the world. Carnegie sold it in 1901 for $480 million to J.P. Morgan, who created U.S. Steel. Carnegie devoted the remainder of his life to large-scale philanthropy, with special emphasis on local libraries, world peace, education and scientific research. His life has often been referred to as a true "rags to riches" story...."

so I'm not sure why they call him that, the good thing is that I know why people call him the captain of industry

I gave you a definition of robber baron a few days ago. They used unethical or questionable practices in their businesses. The drove smaller companies out of business and underpaid and abused their workers.